Molybdenum-base alloys have been used for high temperature applications, such as furnace heating elements, radiant heat shields, susceptor rings for induction furnaces, electrical posts for lamp filaments, high-temperature springs, high-strength fiber reinforcements, high-strength dies, high-strength tubing for ballistic applications, and creep resistant boats for sintering. Molybdenum-base alloys can be strengthened by solid-solution strengthening, precipitation or dispersion strengthening, strain or work hardening and grain size refining, and retaining a fine-grained worked microstructure to high temperatures. For service at elevated temperatures, only the last three or a combination on any of them have the potential for success.
Commercial molybdenum-base alloys generally do not possess sufficient creep and tensile strength to survive operating temperatures above 1315.degree. C. Molybdenum TZM and TZC alloys, strengthened by a combination of alloying, such as solid-solution strengthening, and carbide particles, precipitation strengthening, plus thermomechanical processing, experience substantial degradation of their mechanical properties above 1315.degree. C. This led to the development of an oxide dispersion strengthened (hereinafter "ODS") molybdenum alloy, which provides enhanced stability at temperatures above 1315.degree. C. ODS molybdenum alloy is formed by the following steps:
a slurry is first formed by dispersing molybdenum oxide and a metal salt in an aqueous medium, the metal oxide being selected from nitrates and acetates of lanthanum, cerium or thorium; PA1 the slurry is then heated in the presence of hydrogen to form a molybdenum powder and a metal oxide; PA1 the molybdenum powder is pressed to form a molybdenum compact; PA1 the molybdenum compact is sintered in the presence of hydrogen to form a molybdenum ingot; and PA1 the molybdenum ingot is compacted to reduce the cross-sectional area of the molybdenum ingot and form an ODS molybdenum alloy containing a metal oxide. PA1 (a) forming a slurry comprising molybdenum oxide and a metal salt dispersed in an aqueous medium, the metal salt being selected from nitrates or acetates of lanthanum, cerium or thorium; PA1 (b) heating the slurry in the presence of hydrogen to form a molybdenum powder comprising molybdenum and an oxide of the metal salt; PA1 (c) mixing rhenium powder with the molybdenum powder to form a molybdenum-rhenium powder; PA1 (d) pressing the molybdenum-rhenium powder to form a molybdenum-rhenium compact; PA1 (e) sintering the molybdenum-rhenium compact in the presence of hydrogen or a vacuum to form a molybdenum-rhenium ingot; and PA1 (f) compacting the molybdenum-rhenium ingot to reduce the cross-sectional area of the molybdenum-rhenium ingot and form a molybdenum-rhenium alloy containing said metal oxide. PA1 (a) forming a slurry comprising molybdenum oxide and a metal salt dispersed in an aqueous medium, the metal salt being selected from nitrates or acetates of lanthanum, cerium or thorium made; PA1 (b) heating the slurry in the presence of hydrogen to form a molybdenum powder comprising molybdenum and the metal oxide; PA1 (c) mixing rhenium powder with the molybdenum powder to form a molybdenum-rhenium powder; PA1 (d) pressing the molybdenum-rhenium powder to form a molybdenum-rhenium compact; PA1 (e) sintering the molybdenum-rhenium compact in the presence of hydrogen or under a vacuum to form a molybdenum-rhenium ingot; and PA1 (f) compacting the molybdenum-rhenium ingot to reduce the cross-sectional area of the molybdenum-rhenium ingot and form the molybdenum-rhenium alloy containing the metal oxide.
The creep-rupture life of unalloyed molybdenum at 0.65 T.sub.m (1600.degree. C.) is increased by approximately 4 to 5 orders of magnitude by using the ODS method.
Other attempts to improve the high temperature properties of molybdenum include adding rhenium at amounts up to 50% by weight. U.S. Pat. No. 5,437,744 discloses a molybdenum-rhenium alloy which contains 42 up to less than 45% by weight rhenium. This patent does not disclose a molybdenum-rhenium alloy containing La, Ce, or Th, which has been formed using the ODS method described above. The creep-rupture life of unalloyed molybdenum at 0.65 T.sub.m (1600.degree. C.) is only increased by approximately one order of magnitude with up to 50% by weight addition of rhenium.
While ODS molybdenum alloy provides enhanced stability at elevated temperatures, there is still a great need for further improvements in the high temperature stability of ODS molybdenum.